1. Field of the Invention
The present invention relates to a process for producing a fluorinated polymer having ion-exchange groups. More particularly, it relates to an improved process for producing a fluorinated polymer having carboxylic acid type cation exchange groups by a copolymerization in an aqueous medium.
2. Description of the Prior Arts
It has been known that a cation exchange membrane of a carboxylic acid type fluorinated polymer is used as a membrane for partitioning in a membrane type electrolysis of an aqueous solution of an alkali metal chloride to produce an alkali metal hydroxide having high purity and to attain an operation at high current efficiency and high current density and also to produce an alkali metal hydroxide at high concentration in a cathode compartment.
The inventors have invented that a copolymer having high ion-exchange capacity and high molecular weight can be smoothly produced by copolymerizing a fluorinated olefin and a carboxylic acid type functional monomer in an aqueous medium at a desired ratio of the aqueous medium to the carboxylic acid type functional monomer and a pressure for the copolymerization. This is disclosed in U.S. Pat. No. 4,138,373 and U.K. Pat. No. 1,552,060 and Japanese Unexamined Pat. No. 49090/1978.
In accordance with the inventors' studies, the following problems of the resulting copolymers are found in the copolymerizations of a fluorinated olefin such as tetrafluoroethylene and a carboxylic acid type functional monomer in an aqueous medium.
When the resulting copolymer is molded into a film or a sheet in a heat-melting, the melt-viscosity of the copolymer is too high not to be easily molded. The difficulty of the moldability is not only depending upon the increase of the molecular weight, but also certain modification of a carboxylic acid type functional groups in the copolymerization in an aqueous medium in view of a result of the measurement of the molecular weight.
The inventors have further studied the above-mentioned copolymerization in view of the finding of these facts, and have found the following interest facts.
The copolymer obtained by an emulsion polymerization etc. is heated with an alcohol such as methanol whereby the melt-viscosity of the copolymer can be effectively lowered. The resulting copolymer is easily moldable into a film or a sheet by a melt-molding.